A-C:H ISFET device, manufacturing method, and testing methods and apparatus thereof

ABSTRACT

An a-C:H ISFET device and manufacturing method thereof. The present invention prepares a-C:H as the detection membrane of an ISFET by plasma enhanced low pressure chemical vapor deposition (PE-LPCVD) to obtain an a-C:H ISFET. The present invention also measures the current-voltage curve for different pH and temperatures by a current measuring system. The temperature parameter of the a-C:H ISFET is calculated according to the relationship between the current-voltage curve and temperature. In addition, the drift rates of the a-C:H ISFET for different pH and hysteresis width of the a-C:H ISFET for different pH loops are calculated by a constant voltage/current circuit and a voltage-time recorder to measure the gate voltage of the a-C:H ISFET.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ISFET, and in particular to ahydrogenated amorphous carbon (a-C:H) ISFET device, manufacturingmethod, and method and apparatus to measure hysteresis width and driftrate therewith.

2. Description of the Related Art

ISFETs (Ion Sensitive Field Effect Transistor) are constructed bysubstituting a detecting film for the metal gate on the gate oxide of aconventional MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor).When the ISFET is dipped into a solution, the interfacial potentialbetween the detecting film and the solution influences the semiconductorsurface since only an extremely thin dielectric (that is, the gateoxide) separates the detecting film from the semiconductor surface. Thisinfluences the charge density in the inversion layer of thesemiconductor surface, and thereby modulates the channel current throughthe ISFET. Thus, by utilizing this characteristic, the pH or other ionconcentration in a solution can be obtained from the measurement ofsource/drain current and the gate voltage of the ISFET. The potentialdifference on the interface between the detecting film and the solutionis related to the ion activity in a solution. The hydrogen ion activityin the solution can be measured using different channel currents causedby different interfacial potential differences in various solutions withdifferent hydrogen ion activity.

Patents related to the formation of the ISFET or measurement thereof arelisted hereinafter.

U.S. Pat. No. 5,350,701 discloses a method of measuring the content ofalkaline-group metals, especially the content of calcium ions, utilizingchemosynthesis phosphide group as the detecting film on a gate of anISFET.

U.S. Pat. No. 5,387,328 discloses a bio-sensor using ion sensitive fieldeffect transistor with platinum, wherein an enzyme membrane isimmobilized on the ion-detecting film to determine the concentration ofglucose.

U.S. Pat. No. 5,414,284 discloses a method of fabricating an ISFET andan ESD protective circuit on the same silicon substrate, wherein acapacitor is utilized as an interface between the protective circuit anda sample solution to the DC leakage current.

U.S. Pat. No. 5,309,085 integrates the measurement circuit of a creaturesensor having ISFET on a wafer. The measured circuit has two ISFETdevices, an enzyme ISFET and a reference electrode FET, whose outputsignal can be amplified by a differential amplifier.

U.S. Pat. No. 5,061,976 discloses a carbon thin film on the gate oxideof the ISFET and then a 2,6 xylenol electrolytic polymerization filmformed thereon. The ISFET has the ability to detect hydrogen ions andthe advantages of small floating time, high reliability, andinsensitivity to light. When other film types are covered on the ISFET,other kinds of ions can be detected.

U.S. Pat. No. 5,833,824 discloses an ISFET sensor for detecting ionactivity in a solution, which includes a substrate and an ISFETsemiconductor die. The substrate has a front surface exposed to thesolution, a back surface opposite to the front surface and an apertureextending therebetween. A detecting film of the ISFET is mounted on theback surface such that the gate region is exposed to the solutionthrough the aperture.

U.S. Pat. No. 4,691,167 discloses a method of measuring ion activationin a solution by combining the ISFET, the reference electrode, thetemperature sensor, amplifier circuit and a calculation and memorycircuit. Since the sensitivity is a function of the temperature anddrain current of ISFET and is decided by a variable of gate voltage, thesensitivity can be obtained by calculating formulas stored in memory.

U.S. Pat. No. 5,130,265 discloses a method of fabricating the ISFET withmultiple functions. The method uses siloxanic prepolymer as thesensitive film, mixing the solution, photochemistry treatment and heattreatment.

U.S. Pat. No. 4,660,063 discloses a method of performing both laserdrilling and solid diffusion to form a 3D diode array on thesemiconductor wafer. The laser first drills the wafer, and theimpurities are then diffused from the hole to form a cylindrical PNjunction and complete a non-planar ISFET structure.

U.S. Pat. No. 4,812,220 discloses an ISFET made by fixing the enzyme onthe detecting film to measure the concentration of amino acids in food.The enzyme sensor is miniaturized, and can accurately measure even smallconcentrations.

There are many materials acting as detection membranes of ISFETs, suchas, Al₂O₃, Si₃N₄, Ta₂O₅, a-WO₃, a-Si:H and the like. These thin filmsare deposited by either sputtering or plasma enhanced chemical vapordeposition (PECVD), therefore, the cost of the thin film fabrication ishigher. For commercial purposes, it is important to develop a thin film,with low cost and easy fabrication.

In the ISFET applications, however, many factors such as hysteresis,temperature, and drift behavior affect the accuracy of the measuringresults. Since pH-ISFET is a semiconductor device, it is easilyinfluenced by variations in temperature. The variation of thetemperature leads to a deviation of the measurement. With reference tothe hysteresis behavior, it is related to the change in the pH of thesolution (such as pH_(x)→pH_(y)→pH_(x)→pH_(z)→pH_(x)) and thecorresponding change in the output voltage of the ISFET (such asV_(ox1)→V_(oy)→V_(ox2)→V_(oz)→V_(ox3)). At the same pH, the differencebetween the first output voltage and the final output voltage (such asV_(ox3)−V_(ox1)) is defined as the hysteresis width. For drift behavior,the drift rate is defined as the change in the gate voltage per unittime under conditions in which the source-drain current is stable andthe temperature is constant after the intrinsic response of the pH-ISFETis completed. Hence, there is a need to measure the three effects toprevent error.

SUMMARY OF THE INVENTION

In view of this, an object of the present invention is to provide ana-C:H gate ISFET. The present invention forms the a-C:H layer as thedetection membrane of the ISFET by plasma enhanced low pressure chemicalvapor deposition (PE-LPCVD).

Another object of the present invention is to provide a method ofmeasuring temperature parameters of an ISFET. In the present invention,the sensitivities of the, ISFET at different temperatures are obtainedby the source-drain current and gate voltage of the ISFET in a solution,such that temperature parameters (temperature coefficient of thesensitivity) of the ISFET are obtained.

In the method of measuring the temperature parameters of an ISFETaccording to the present invention, the detecting film is immersed in abuffer solution, and, then, at a predetermined temperature, the pH ofthe buffer solution is changed to measure and record the source-draincurrent and the gate voltage of the ISFET to obtain a curve. Thetemperature parameters at the predetermined temperature are obtained byselecting a fixed current from the curve. The temperature parameters atother temperatures are obtained by changing the temperature of thebuffer solution and the steps of measuring, recording and selecting.

Another object of the present invention is to provide a method ofmeasuring the hysteresis width and drift rate of the a-C:H ISFET to usethe reverse compensation method to obtain an accurate output value.

In the method of measuring the hysteresis width of an a-C:H ISFETaccording to the present invention, first, the drain-source current andthen the drain-source voltage are fixed by a constant voltage/currentcircuit, and the a-C:H ISFET is immersed in a buffer solution. Thegate-source output voltage of the a-C:H ISFET is recorded by avoltage-time recorder, and the pH of the buffer solution is changed. Thesteps of immersing and recording are then repeated to obtain thegate-source output voltages of the ISFET immersed in the buffer solutionwith different pH. The hysteresis width is the voltage deviation betweenstarting pH and ending pH.

In the method of measuring the drift rate of an a-C:H ISFET according tothe present invention, first, the drain-source current and then thedrain-source voltage are fixed by a constant voltage/current circuit,and the a-C:H ISFET is immersed in a buffer solution. The gate/sourceoutput voltage of the a-C:H ISFET during a constant period is recordedby a voltage recorder. The pH of the buffer solution is changed and thesteps of immersing and recording are repeated to obtain the gate-sourceoutput voltages of the ISFET immersed in the buffer solution withdifferent pH. The drift rate is the slope of the gate-source outputvoltage with respect to time.

Another object of the present invention is to provide an apparatus tomeasure the hysteresis width and the drift rate. The apparatus ofmeasuring the hysteresis width and the drift rate has an a-C:H ISFET, abuffer solution to contact the ISFET, a light-isolation container toload the buffer and to isolate light, a heater to heat the buffersolution, a constant current/voltage measuring device coupled to thesource and drain of the a-C:H ISFET, and a voltage-time recorder torecord the output voltage of ISFET.

Further scope of the applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more fully understood by reading thesubsequent detailed description and examples with references made to theaccompanying drawings, which are given by way of illustration only, andthus are not limitative of the present invention, and wherein:

FIGS. 1 a to 1 c are cross-sections of the processes of the presentinvention;

FIG. 2 is a cross-section of an ISFET with gate consisting of a-C:H;

FIG. 3 is a schematic structural diagram of the apparatus to measure thetemperature parameter of the a-C:H ISFET according to the presentinvention;

FIG. 4 is a schematic cross-section of the a-C:H ISFET according to thepresent invention;

FIG. 5 shows a curve related to the source/drain current and the gatevoltage of the a-C:H ISFET at 25° C. according to the present invention;

FIG. 6 shows a curve related to the gate voltage of the a-C:H ISFET andthe pH;

FIG. 7 shows a curve related to the sensitivities of the a-C:H ISFET andthe temperature according to the present invention;

FIG. 8 shows a schematic structure diagram of the apparatus to measurethe hysteresis width and drift rate of the a-C:H ISFET according to thepresent invention;

FIG. 9 shows a schematic diagram of the constant voltage current circuitaccording to the present invention;

FIG. 10 shows the relationship between the hysteresis width and time inpH=6→2→6→10→6 order; and

FIG. 11 shows the relationship between the drift rate of the a-C:H ISFETand the pH.

DETAILED DESCRIPTION OF THE INVENTION

The a-C:H gate ion sensitive field effect transistor (a-C:H gate ISFET)according to the present invention is illustrated in FIGS. 1 a to 1 c.

As shown in FIG. 1 a, a P-type (100) semiconductor substrate 100 with aresistivity ranging from 8 to 12 Ω-m is provided. A pad oxide layer 102consisting of silicon dioxide with a thickness of 5000Å is formed on thesubstrate 100 by wet oxidation. A first photoresist pattern (not shown)is formed on the pad oxide layer 102 by conventional photolithography. Adummy gate 103 is formed to define the subsequent gate area, using thephotoresist pattern as a mask to a portion of the pad oxide layer 102.

Impurities are then implanted into the semiconductor substrate 100 toform a source/drain 104 beside the dummy gate 103, using the dummy gate103 as a mask. For example, the impurities implanted herein are boronions with a dose of 10¹⁵ cm⁻².

As shown in FIG. 1 b, the dummy gate 103 is removed, namely, the padoxide layer 102 and the first photoresist pattern are removed by wetetching. An insulating layer 106 consisting of silicon dioxide with athickness of about 1000Å is then formed on the semiconductor substrate100. A second photoresist pattern (not shown) is formed on theinsulating layer 106 by photolithography. Next, the insulating layer 106outside the gate area is removed by the second photoresist pattern as amask. The residual insulating layer within the gate area is used as agate oxide layer. Subsequently, the second photoresist layer is removed.

A a-C:H layer 108 is then formed on the insulating layer 106 by plasmaenhanced low pressure chemical vapor deposition (PE-LPCVD). The step offorming the a-C:H layer 108 on the gate oxide layer 106 by plasmaenhanced low pressure chemical vapor deposition (PE-LPCVD) isillustrated as follows. First, the semiconductor substrate 100 with thegate oxide layer 106 is disposed into a PE-LPCVD system. During thisprocess, the base pressure of the PE-LPCVD system is adjusted at least10⁻⁶ torr. Next, the temperature of the semiconductor substrate 100 ismaintained between 140° C.˜160° C. (preferably 150° C.). A mixing gasconsisting of methane and hydrogen is then fed into PE-LPCVD system,wherein, in the mixing solution, the ratio of methane and hydrogen isabout 30 to 70. Further, the flow ratio of the mixing gas is controlledbetween 6 to 10 standard cubic centimeter per minute (SCCM), preferably8 SCCM. The process pressure of the PE-LPCVD is maintained between 0.08to 0.1 torr, preferably 0.09 torr. Finally, the RF power supply is thenturned on, and the RF power thereof is set between 145 W to 160 W,preferably 150 W, for a time interval. After that, the a-C:H gate layer108 is obtained on the oxide layer 106 for the a-C:H ISFET.

For example, the a-C:H layer 108 with a thickness of least 1000Å isformed on the insulating layer 106. A two-layer gate consisting of thegate oxide layer 106 and the a-C:H layer 108 is fabricated. Thus, ana-C:H ISFET is obtained. The a-C:H ISFET has a channel length of about50 μm and a channel width of about 1000 μm. Thus, the aspect ratio(channel width/channel length) of the present a-C:H ISFET is 20.

Next, as shown in FIG. 1 c, an interconnecting process is performed toobtain the ion sensitive field effect transistor (ISFET) usingconventional interconnect steps for MOS. Thus, an insulating layer 100is formed on the source/drain 104, and a metal wire 112 is formed on theinsulating 110 by etching and sputtering. Finally, a sealing layer 114consisting of the insulator is formed to seal the metal wire excludingthe a-C:H layer 108. For example, the metal wire 112 can be aluminum,and the sealing layer 114 can be epoxy resin.

The use of plasma enhanced low pressure chemical vapor deposition toform the a-C:H detection membrane provides a simple process and easilyformed a-C:H detection membrane on a large area surface of thesubstrate.

FIG. 2 is a cross-section of the a-C:H ISFET according to the presentinvention. The structure of this ISFET is similar to that of MOSFET. Thedifference between the ISFET and MOSFET is that the metal gate of theMOSFET is replaced by an a-C:H detection membrane 35, an aqueoussolution 36 and reference electrode 38. The circuits are formed bycontacting the metal wire 31, preferably consisting of Al, with thesource/drain 33. Since the a-C:H detection membrane 35 contacts thedetected solution 36, the whole device in addition to the a-C:Hdetection membrane 35 must be enclosed by a sealing layer 37 consistingof a material with good insulating property, such as epoxide resin. Thereference electrode 38 provides a detecting base.

The a-C:H detection membrane 35 of the ISFET is immersed in the solution36 during operation, such that the point of transformation from chemicalequivalence into electrical equivalence within the ISFET occurs withcontact between the a-C:H detection membrane 35 and the aqueous solution36. The reaction mechanism of the ionic activity within the solution isthe interface potential obtained from the interface between the aqueoussolution 36 and the a-C:H detection membrane 35 immersed in the aqueoussolution 36. The interface potential varies with the ionic activities ofvarious aqueous solutions. In addition, the interface potentialregulates the channel conduction of the ISFET and results in the changeof current within the source/drain 33.

FIG. 3 shows a systematic structure diagram according to the presentinvention. An ISFET using a-C:H as a detection membrane (called a-C:HISFET) is immersed in a buffer solution 2 such as the phosphate buffersolution that is stored in a container (not titled). The source/drain(not shown) of the a-C:H ISFET 1 connects a test fixer 3 through twoconnecting wires 331 and 321, respectively, to convey the electricalsignals obtained by measuring the source/drain to a current/voltagemeasuring device 4, such as the Keithley-236 current/voltage measuringdevice for data processing.

Also, a reference electrode 5 is disposed in the buffer solution 2, withone end connected to the test fixer 3 through the connecting wire 331. Aplurality of heaters 6 is disposed outside the container and connectedto a PID temperature controller 7. A thermometer 8 connected to the PIDtemperature controller 7 detects the temperature of the buffer solution2. The above-mentioned elements such as the buffer solution 2, theelements contacting the buffer solution 2 and the heater 6 are placed ina light-isolating container 9 to protect the measured data from theeffect of light.

It should be noted that the interfacial potential between the a-C:Hmembrane and the solution and the characteristic difference of chargedensity in the reverse layer of the semiconductor surface are used tomeasure needed data (such as the source-drain current or the gatevoltage) and thus obtain the temperature parameters of the ISFET.

FIG. 4 is a schematic cross-section of the a-C:H ISFET according to thepresent invention. The a-C:H ISFET is formed on a semiconductorsubstrate 20 such as a p-type silicon substrate. A pair of source/drainregions 21 separated from each other is formed approaching the topsurface of the semiconductor substrate 20 and each region 21 isconnected to the test fixer 3 outside through an aluminum contact plug22 and an aluminum wire 23. On the semiconductor substrate 20 betweenthe two drain/source regions 21, a gate oxide 24, such as a siliconoxide layer, and an a-C:H detection membrane 25 are formed sequentially.An epoxy resin 26 seals the device but exposes the a-C:H detectionmembrane 25. As well, a metal-aluminum layer 27 is formed at the bottomof the semiconductor substrate 20 to shield light and decrease itsinfluence on charge carriers.

With reference to apparatus shown in FIG. 3 and FIG. 4, the temperatureparameters of the a-C:H ISFET of the present invention can be obtainedaccording to a method as follows. First, with regard to the measurementof the sensitivity, the detection membrane of the a-C:H ISFET contactswith the buffer solution. The temperature of the buffer solution isfixed, such as at 25° C., and the pH of the buffer solution is changedat the same time. The curve related to the source-drain current and thegate voltage of the a-C:H ISFET is measured and recorded by theKeithley-236 current/voltage measuring device. FIG. 5 shows curvesrelated to the statistical results, and both the source-drain currentand the gate voltage of the a-C:H ISFET rise as the pH of the bufferresolution increases.

Next, a fixed current of the curve (like 80 μA) is selected to obtain acurve related to the gate voltage and the pH at a specific temperature(like 25° C.) as shown in FIG. 6, wherein the sensitivity of the a-C:HISFET at 25° C. is 57.36 mV/pH. It is found that the gate voltage of thea-C:H ISFET is in direct proportion to the pH of the buffer solution andthe slope of the curve is the sensitivity of the a-C:H ISFET at thespecific temperature.

Moreover, in order to measure the sensitivity of the a-C:H ISFET atdifferent temperatures, only the temperature of the buffer solutionneeds to be changed, such as between 5° C.˜55° C., with the above steprepeated at each temperature. Table 1 shows the sensitivity of the ISFETat the different temperatures.

TABLE 1 the sensitivity of the ISFET at the different temperaturesSensitivity Temperature (° C.) (mv/pH) 5 53.68 15 55.43 25 56.92 3559.23 45 61.5 55 62.8

A curve showing the obtained sensitivities at different temperatures isshown in FIG. 7, wherein the sensitivity is in direct proportion to therising temperature and the slope of the curve is about 0.2032 mV/pH° C.Namely, the temperature parameter of the a-C:H ISFET is about 0.2032mV/pH° C.

FIG. 8 shows a schematic diagram to measure the hysteresis width and thedrift rate of an ISFET with a-C:H as a detection membrane according tothe present invention. An ISFET 81 with a-C:H as a detection membrane(called a-C:H ISFET) is immersed in a buffer solution 82 such as astandard buffer solution in a container (not labeled). A drain/source(not shown) of the a-C:H ISFET 81 is connected to a constantvoltage/current circuit 83 (such as a negative feedback circuit) throughtwo wires 811 and 812. The drain-source voltage and the drain-sourcecurrent of the a-C:H ISFET 81 are fixed by the constant voltage/currentcircuit 83.

A reference electrode 84 is disposed in the buffer solution 82, whereinone end of the reference electrode 84 is connected to the constantvoltage/current circuit 83 through a wire 841. A heater 85 disposedoutside the container is connected to a PID(Proportional-Integral-Derivative) temperature controller 86. Both theheater 85 and the PID temperature controller 86 keep the buffer solution82 at a constant temperature (preferably 25° C.) detected by athermocouple 87 connected to the PID temperature controller 86. Theabove buffer solution 82, every device connected thereto, and the heater86 are disposed in a light-isolating container 88 to reduce the effectof light on the measuring results.

The constant voltage/current circuit 83 is connected to acurrent/voltage measuring device 89 composed of two digital multimetersdetecting whether the source-drain current and the source-drain voltageof the a-C:H ISFET 81 move towards stability. Also, the constantvoltage/current circuit 83 is connected to a voltage-time recorder 810for setting and recording the output voltages during each recordingperiod.

FIG. 9 shows a schematic diagram of the constant voltage/current circuit83 according to the present invention. The constant voltage/currentcircuit 83 is connected to the source/drain of the a-C:H ISFET 81through the wires 811 and 812, and is connected to the referenceelectrode 84 through the wire 841. The source-drain voltage is fixed ata constant value (preferably 0.2V) by adjusting the variable resistanceR1. The source-drain current is fixed at a constant value (preferably 80μA). In this case of the negative feedback circuit, the output voltageand the gate voltage are reduced and finally the drain-source currentI_(DS) is reduced when the increasing drain-source current I_(DS)increases the source voltage. The circuit 83 has advantages ofsimplicity, low cost, ease of operation and no need for adjustment ofthe measuring point.

Next, returning to FIG. 4, a schematic cross-section of the a-C:H ISFETaccording to the present invention, the a-C:H ISFET is formed on asemiconductor substrate 20 such as a p-type silicon substrate. In thiscase, a pair of source/drain regions 21 separated from each other areformed approaching the top surface of the semiconductor substrate 20,and are connected to constant voltage/current circuit by an aluminumcontact plug 22 and an aluminum wire 23. On the semiconductor substrate20 between the two drain/source regions 21, a gate oxide 24, such as asilicon oxide layer, and an a-C:H detection membrane 25 are formedsequentially. An epoxy resin 26 seals the device but exposes the a-C:Hdetection membrane 25. As well, a metal-aluminum layer 27 is formed atthe bottom of the semiconductor substrate 20 to decrease thechannel-adjusting effect.

Hereinafter, a method to measure the hysteresis width and the drift rateof the a-C:H ISFET in detail is described with reference to FIGS. 2, 8and 9.

With reference to the method of measuring the hysteresis width of thea-C:H ISFET, first, the drain-source current and the drain-sourcevoltage of the a-C:H ISFET 81 are fixed by the constant voltage/currentcircuit (negative feedback circuit) 83. In this step, the a-C:H ISFET 81and the reference electrode 84 are connected to the constantvoltage/current circuit 83, and are immersed in the solution. Next, thedrain voltage V_(D) of the a-C:H ISFET 81 is set at 0.2V by adjustingthe variable resistant R1 and measurement by one digital multimeter.Also, the drain-source current I_(DS) is set at 80 μA by adjusting thevariable resistant R2 and measurement by the other digital multimeter.The a-C:H ISFET 81 is then placed in a standard solution to maintainstability.

After, the a-C:H ISFET 81 is immersed in a buffer solution. Next, thevoltage-time recorder records the gate-source output voltages of a-C:HISFET 81. The hysteresis width of the a-C:H ISFET 81 in accordance withthe pH of the buffer solution is then measured. In the presentinvention, hysteresis width is measured in pH=6→2→6→10 order, namelypH=6-5-4-3-2-3-4-5-6-7-8-9-10-9-8-7-6, wherein each measuring result isachieved at the time the pH is changed for one minute, the loop time is1020 seconds, and each time the pH changes by one unit. Particularly,choosing the pH=6-2-6-10-6 order measures hysteresis within the pH rangebetween 1 and 9. Also, the hysteresis within the pH range between 1 and9 are measured in the same way, wherein each result is at the time whenthe pH is changed for two minutes and four minutes, the loop time is2040 seconds and 4080 seconds. Table 2 shows the hysteresis width of thea-C:H ISFET in pH=6→2→6→10→6 order at different loop time. By the samemethod, all of the hysteresis widths at different pH values can bemeasured, which is helpful in performing the reverse compensationmethod.

TABLE 2 the hysteresis width of the a-C:H ISFET in pH = 6 → 2 → 6 → 10 →6 order at different loop time Loop time (seconds) Hysteresis width (mV)1020 2.38 2040 3.86 4080 4.5 8160 5.4 16320 7.22

The relationship between the hysteresis width, the pH and time measuredin pH=6→2→6→10→6 order are shown in FIG. 10. As shown in FIG. 10, thehysteresis width of the a-C:H ISFET increases as the loop timeincreases.

Next, with reference to the drift rate, the drain-source current and thedrain-source voltage of the a-C:H ISFET are fixed by the constantvoltage/current circuit (negative feedback circuit) 83. The a-C:H ISFET81 and the reference electrode 84 are then connected to the constantvoltage/current circuit 83, and immersed in the solution. Next, thedrain-source current I_(DS) of the ISFET 81 is set by adjusting thevariable resistor R2 and measurement by one digital multimeter. Also,the drain-source voltage V_(DS) is set at 0.2V by adjusting the variableresistor R1 and measurement with the other digital multimeter.Afterwards, the a-C:H ISFET is immersed in the buffer solution for aperiod of time. Finally, the gate-source output voltage is recorded bythe voltage-time recorder, thereby obtaining the drift rate of theISFET.

It should be noted that current generated by illumination affects thedrift rate. Hence, the drain-source current should be adjusted between10 μA and more than one hundred μA to reduce the illumination effect. Aswell, stability is easily affected by temperature when the drain-sourcecurrent I_(DS) is extremely large. Accordingly, the drain-source currentis preferably set at 10˜300 μA.

Table 3 shows the drift rates of the a-C:H ISFET at pH 1˜10, and FIG. 11shows the relationship between the drift rate and the pH. The drift rateof the a-C:H ISFET is obtained from the slope of a curve whose timeparameter is more than the fifth hour.

TABLE 3 the drift rates of the a-C:H ISFET at pH 1˜10 pH Drift rate(mV/h) 1 0.44 2 0.5 3 0.55 4 0.68 5 1.06 6 1.3 7 1.56 8 1.8 9 2.12 102.38

It is believed that the drift behavior is more obvious when the pH islarge. Also, when the data approximately forms a line, the drift ratesat any other pH can be estimated. This is useful when performing reversecompensation.

The method of forming the a-C:H ISFET according to the present inventionis simple, has a low cost and novel technology. The measuring methodsand apparatus can accurately measure hysteresis width and drift rate ofthe a-C:H ISFET, and also hysteresis width and the drift rate of theISFETs with other types of detection membranes.

While the invention has been described by way of example and in terms ofthe preferred embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements (aswould be apparent to those skilled in the art). Therefore, the scope ofthe appended claims should be accorded the broadest interpretation toencompass all such modifications and similar arrangements.

1. A method of measuring the hysteresis width of an ISFET with a-C:H asa detection membrane, comprising the steps of: fixing the drain-sourcecurrent and the drain-source voltage of the ISFET by a constantvoltage/current circuit; immersing the detection membrane in a buffersolution; recording the gate/source output voltage of the ISFET by avoltage-time recorder; and changing the pH of the buffer solution andrepeating fixing, immersion and recording to measure the hysteresiswidth of the ISFET.
 2. The method as claimed in claim 1, wherein thehysteresis width is the change in the gate/source output voltage fromthe first measuring point to the final measuring point.
 3. The method asclaimed in claim 1, wherein the source-drain current is fixed at 80 μA,and the drain-source voltage is fixed at 0.2V.
 4. The method as claimedin claim 1, further comprising immersing the ISFET with a-C:H as adetection membrane in a standard solution to maintain stability prior toimmersing the detection membrane in the buffer solution.
 5. The methodas claimed in claim 1, wherein the pH is changed from pH=6 to pH=2, topH=6, to pH=10, and to pH=6.
 6. The method as claimed in claim 5,wherein each pH level of the buffer solution is fixed for one minute.